Deep-trap ultraviolet persistent phosphor for advanced optical storage application in bright environments

Extensive research has been conducted on visible-light and longer-wavelength infrared-light storage phosphors, which are utilized as promising rewritable memory media for optical information storage applications in dark environments. However, storage phosphors emitting in the deep ultraviolet spectral region (200–300 nm) are relatively lacking. Here, we report an appealing deep-trap ultraviolet storage phosphor, ScBO3:Bi3+, which exhibits an ultra-narrowband light emission centered at 299 nm with a full width at half maximum (FWHM) of 0.21 eV and excellent X-ray energy storage capabilities. When persistently stimulated by longer-wavelength white/NIR light or heated at elevated temperatures, ScBO3:Bi3+ phosphor exhibits intense and long-lasting ultraviolet luminescence due to the interplay between defect levels and external stimulus, while the natural decay in the dark at room temperature is extremely weak after X-ray irradiation. The impact of the spectral distribution and illuminance of ambient light and ambient temperature on ultraviolet light emission has been studied by comprehensive experimental and theoretical investigations, which elucidate that both O vacancy and Sc interstitial serve as deep electron traps for enhanced and prolonged ultraviolet luminescence upon continuous optical or thermal stimulation. Based on the unique spectral features and trap distribution in ScBO3:Bi3+ phosphor, controllable optical information read-out is demonstrated via external light or heat manipulation, highlighting the great potential of ScBO3:Bi3+ phosphor for advanced optical storage application in bright environments.


Characterization
The crystal structure of the as-synthesized phosphors was characterized via powder X-ray diffraction (XRD, DMAX-2500PC, Rigaku) with Cu Kα irradiation (λ = 1.5418Å) at a scanning speed of 10° min −1 .The morphology and elemental distribution of the samples were recorded by a JSM-7800F field-emission scanning electron microscope (SEM).The photoluminescence (PL), persistent luminescence (PersL), and photostimulated luminescence (PSL) properties were measured using an FLS1000 spectrofluorometer (Edinburgh Instruments) equipped with a photomultiplier tube detector (PMT, 200-900 nm) and a 400 W Xe lamp as the excitation source.The X-ray source (MOXTEK MagPro) was used as an excitation source.The low-temperature and high-temperature experiments were realized with an OptistatDN cryostat (Oxford Instruments) equipped with a MercuryiTC temperature-controlled system.Thermoluminescence (TL) spectra were conducted by using an SL18 thermoluminescence setup (Guangzhou Rongfan Science and Technology Co., Ltd; heating rate, 4 K s −1 ).The ultraviolet afterglow images were taken by the ultraviolet camera (Ofil Scalar), which are overlay images after the addition of a ultraviolet image onto a visible image.The PSL measurements and imaging experiments are carried out in an indoor-lighting environment (under the illumination of a Bull white LED lamp).The electron paramagnetic resonance (EPR) spectra were obtained using a Bruker A300 spectrometer.

Parameter settings
The initial atomic positions and symmetry information of the ScBO3 crystal structure were taken from the Inorganic Crystal Structure Database and the periodic 2 × 2 × 1 supercell containing 120 atoms (24 Sc atoms, 24 B atoms and 72 O atoms) was used to simulate.Using the Vienna Ab initio simulation package (VASP), 1 theoretical simulations were carried out using the density functional theory (DFT), and the generalized gradient approximation (GGA)-Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was adopted.The Sc (3s 2 3p 6 4s 1 3d 2 ), B (2s 2 2p 1 ), O (2s 2 2p 4 ) and Bi (5d 10 6s 2 6p 3 ) were treated as valence electrons, and their interactions with the cores were described by the projector augmented wave (PAW) method.The energy change and the Hellmann-Feynman forces on atoms were set to 10 -5 eV and 0.01 eV Å -1 , respectively.The plane-wave cut-off energy was set at 550 eV.For k-point integration within the first Brillouin zone, a 3 × 3 × 3 Monkhorst-Pack grid was selected.

Formation energy and charge transition levels
The value of defect formation energy reflects the ease of defect formation and the stability of the defect system.The formation energy of a defect in charge state q is defined by: where   [  ] and   [ℎ𝑜𝑠𝑡] are the total energies of the supercell with or without defect D with charge q;   means the change in number for atom i due to defect D, which is added to (  >0)or removed from (  <0) the perfect supercell;   is the chemical potential for atom i;   is the electron Fermi energy;   is the energy state of the valence band maximum.
The atomic chemical potentials in eq (1) are correlated in the thermodynamic equilibrium: where   3 is the total energy of one formula unit of ScBO3, and   ,   ,   are the chemical potentials of Sc, B and O components, respectively.As the phosphors were synthesized in air, and 20% excess B was added during the synthesis, the atomic potentials were determined by the following expressions: The thermodynamic charge-transition levels within the band gap correspond to the Fermi level position at which a transition occurs from one charge state (q) to another (q′).The level position ε(q/q') regarding the host VBM can be deduced from eq (1) as: where ∆  (q or q′; E F = 0) is the formation energy of the defect in charge state q or q′ when the Fermi level is set at 0 eV.

Fig. S1
Fig. S1 Crystal structure and morphology of the ScBO3:Bi 3+ phosphor.a A schematic diagram of the crystal structure of the ScBO3 host.b XRD patterns of ScBO3:x%Bi 3+ phosphors (0 < x ≤ 1).c The Rietveld refinement of the ScBO3:Bi 3+ phosphor.d EDS spectrum of the ScBO3:Bi 3+ phosphor.The insets show the SEM images and corresponding EDS mapping images of the ScBO3:Bi 3+ phosphor.

Fig. S2 a
Fig. S2 a The emission spectrum of the ScBO3:Bi 3+ phosphor with hv (eV) as a horizontal coordinate.b Photoluminescence emission spectra of the ScBO3:x%Bi 3+ (0 < x ≤ 1) phosphors at room temperature.The emission spectra were obtained under the excitation of 280 nm UV light.

Fig. S3 a
Fig. S3 a The normalized emission and excitation spectra of the ScBO3:Bi 3+ phosphor at 77 K and room temperature.b The measured luminescence decay curve of the ScBO3:Bi 3+ phosphor at 77 K.

Fig
Fig. S4 a, b The calculated band structure and total DOS and partial DOS of the ScBO3 host.

Fig. S5
Fig. S5 The Gaussian fitting of the thermoluminescence spectrum of the ScBO3:Bi 3+ phosphor and the value of the peaks are shown in the figure.The sample was pre-irradiated with an X-ray for 25 min.

Fig.
Fig. S6 a, b TL curves of the ScBO3:Bi 3+ phosphors with different Bi 3+ doping concentrations and different excitation durations.The TL curves were acquired at 60 s decay after ceasing X-ray irradiation.

Fig. S8
Fig. S8 Emission spectrum of the used white LED light source.

Fig. S9 a
Fig. S9 a Ultraviolet luminescence decay curves of the ScBO3:Bi 3+ phosphor monitored at 299 nm after irradiation by X-ray for 25 min.The decay curves were measured under the stimulation of the 6 mW 808 nm laser and 1000 lux white LED illumination, respectively.b TL curves of the pre-irradiated ScBO3:Bi 3+ phosphor after 6 h decay under different ambient conditions (darkness, 1000 lux white LED, 6 mW 808 nm laser).

Fig. S10
Fig. S10 Ultraviolet luminescence decay curve of the ScBO3:Bi 3+ phosphor monitored at 299 nm under the stimulation of the 808 nm laser (6 mW) after irradiation by X-ray for 25 min.

Fig
Fig. S11 a-c Ultraviolet luminescence decay curves of the ScBO3:Bi 3+ phosphor at different temperatures after irradiated by X-ray for 25 min

Fig. S13
Fig. S13 The considered four different interstitial sites in the ScBO3 crystal structure.

Fig. S14
Fig. S14 EPR spectra of the unirradiated phosphor and the sample after X-ray irradiation.

Fig. S15 a
Fig. S15 a XRD patterns of YBO3:x%Bi 3+ phosphors (0 < x ≤ 1).b TL curves of the YBO3:x%Bi 3+ phosphors (0 < x ≤ 1).c PersL decay curves of the YBO3:Bi 3+ phosphor upon darkness and 600 lux illumination.d, e Time-dependent TL curves of the YBO3:Bi 3+ phosphor upon darkness and 600 lux white LED illumination.TL curves were acquired by monitoring at 312 nm over the range of 298-650 K. f The emission spectra acquired at 10 min decay upon darkness and 600 lux white LED illumination.g Decay curves of the YBO3:Bi 3+ phosphor at room temperature under photo-stimulation of different laser after irradiated by X-ray.h TL curves of the YBO3:Bi 3+ phosphor after 30 min decay in darkness with photo-stimulation of different lasers.i The integral of the enhanced luminescence intensity (IPSL) and the decreased TL intensity (ITL) along with the effectiveness factor (E) of the pre-irradiated YBO3:Bi 3+ phosphor after photo-stimulation of different lasers.

Fig. S16
Fig. S16 The schematic of the optical information encryption.a The schematic of the input A (1100) and input B (1010).b The read-out process of the four-bit binary optical information.c Photograph of the phosphor discs array was read-out by the ultraviolet camera at room temperature under 453 K (inset i), indoor white LED light (300 lux) (inset ii) and darkness + 635 nm laser (10 mW) (inset iii).d The 16 results of the optical information read-out based on the × 2 unit array.

Fig
Fig. S17 a, b The decay curves monitored at 299 nm under continuous X-ray irradiation and repeated cycles of X-ray excitation.

Fig. S18
Fig. S18 Ultraviolet images of the ScBO3:Bi 3+ phosphor discs underwater recorded by the ultraviolet camera upon (ii) darkness and (iii) white LED illumination at room temperature.

Fig.
Fig. S19 a, b TL curves of the ScBO3:Bi 3+ phosphor under thermal bleaching of 623 K for 30 s and optical bleaching of the blue LED (460-465nm) for 5 min.

Fig. S20
Fig. S20 Stability and repeatability of write-erase of the ScBO3:Bi 3+ phosphor.TL intensity of the ScBO3:Bi 3+ phosphor by alternating a X-ray irradiation for 25 min and thermal bleaching at 623 K for 30 s, and b X-ray irradiation for 25 min and optical bleaching (blue light, 460-465 nm) for 5 min as a function of cycle numbers.